Precast Inc. - July/August 2008 - (Page 13) FIGURE 1 318-08 Section 10.5.4 requires a minimum amount of steel (As, min) for slabs to equal the requirements of Section 7.12 for temperature steel. Section 7.12 requires that: As ≥ 0.002 when grade 40 or 50 deformed bars are used bw x t As ≥ 0.0018 when grade 60 deformed bars or welded wire bw x t fabric are used As = area of steel reinforcement (in.²) bw = width of section considered (in.) t = thickness of slab (in.) As = area of steel reinforcement (in.²) bw = width of section considered (in.) d = distance from extreme compression fiber to centroid of longitudinal tension reinforcement (in.) ρb = α x[ .003 ] x f’c [(fy / Es) + .003] fy α = coefficient based on strength of concrete f’c = compressive strength of concrete (psi) fy = specified yield strength of reinforcement (psi) Es = 29,000,000 psi (modulus of elasticity of steel) α = 0.72 for f’c ≤ 4,000 psi α = 0.68 for f’c = 5,000 psi α = 0.64 for f’c = 6,000 psi f’c (psi) 2,000 fy 40,000 0.025 3,000 0.037 0.021 4,000 0.049 0.028 5,000 0.058 0.034 6,000 0.066 0.038 Maximum reinforcing steel Currently, ASTM Standards refer to ACI 318 for maximum requirements allowed for reinforcing steel. If an excessive amount of steel is placed in a concrete beam or slab, the compression capacity of the concrete will be reached before the steel has a chance to yield (stretch like a rubber band). The result will be a catastrophic collapse without warning. On the other hand, the right amount of steel will permit a large deflection of the beam or slab before failure will occur. The deflection will be noticed and people will have a chance to get to safety. For this reason, ACI 318-08 Section 10.3.3 limits the amount of reinforcing steel in flexural members to: ρ ≤ .75 x ρb where ρ = As bw x d (psi) 60,000 0.014 Table of values for ρb Concrete protection of reinforcement The strength of a slab or beam is dependent on the amount of steel and the amount of concrete where the distance from steel (center line) to the concrete surface in compression is the effective depth (d). But what about the distance from the steel to the concrete surface in tension? A small amount of concrete is JULY/AUGUST 2008 | WWW.PRECAST.ORG ρb = ratio of As to bd producing balanced strain conditions 13 http://www.precast.org
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